Light emitting device and manufacturing method thereof

ABSTRACT

A light emitting device of the invention includes a thin film transistor, an insulating layer covering the thin film transistor, an electrode which is electrically connected to the thin film transistor through a contact hole formed on the insulating layer, and a light emitting element formed by interposing a light emitting layer between a first electrode which is electrically connected to the electrode and a second electrode. The light emitting device further includes a layer formed of a different material from that of the insulating layer only between the electrode and the first electrode over the insulating layer and the insulating layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device which is manufacturedby using an element having a light emitting material interposed betweenelectrodes and emitting light by applying an electric current betweenthe electrodes (light emitting element).

2. Description of the Related Art

In recent years, a thin lightweight display using a light emittingelement has been actively developed. The light emitting element ismanufactured by interposing a material which emits light by applying anelectric current between a pair of electrodes. Since the light emittingelement itself emits light unlike in the case of liquid crystal, a lightsource such as back light is not required, and the element is very thin.Therefore, it is extremely advantageous to manufacture a thinlightweight display.

However, one background of not reaching practical use yet while havingsuch big advantages is a problem of reliability. The light emittingelement often deteriorates due to moisture (water) and has adisadvantage of being hard to obtain long-term reliability. The lightemitting element which is deteriorated due to water causes a decrease inluminance or does not emit light. It is conceivable that this causes adark spot (black spot) and shrinkage (a decrease in luminance from anedge portion of a display device) in a display device using the lightemitting element. Various countermeasures are suggested to suppress suchdeterioration (for example, Reference 1: Japanese Patent Laid-Open No.H9-148066, and Reference 2: Japanese Patent Laid-Open No. H7-169567).

However, these countermeasures are insufficient and have few effects ona defect that a region where emission intensity is decreased isgradually increased with time. A major reason for such a defect is aninfluence of water remaining inside a light emitting device besideswater entering from outside. Additional countermeasures are desired.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a structure and amanufacturing method of a light emitting device, which reduces theamount of water remaining in the light emitting device.

It is another object of the invention to provide a structure and amanufacturing method of a light emitting device, which suppresses thedeterioration of a light emitting element due to water remaining in thelight emitting device.

A light emitting device of the invention includes a thin filmtransistor, an insulating layer covering the thin film transistor, anelectrode which is electrically connected to the thin film transistorthrough a contact hole formed on the insulating layer, and a lightemitting element formed by interposing a light emitting layer between afirst electrode which is electrically connected to the electrode and asecond electrode. The light emitting device further includes a layerformed of a different material from that of the insulating layer onlybetween the electrode and the first electrode and the insulating layer.

Another light emitting device of the invention includes a thin filmtransistor, an insulating layer covering the thin film transistor, anelectrode which is electrically connected to the thin film transistorthrough a contact hole formed on the insulating layer, and a lightemitting element formed by interposing a light emitting layer between afirst electrode which is electrically connected to the electrode and asecond electrode. The light emitting device further includes a layerformed of a different material from that of the insulating layer onlybetween the electrode and the insulating layer.

Another light emitting device of the invention includes a thin filmtransistor, a first insulating layer covering the thin film transistor,and an electrode which is formed over the first insulating layer and iselectrically connected to the thin film transistor through a contacthole formed on the first insulating layer. A wiring and a firstelectrode of a light emitting element are formed over a secondinsulating layer which is formed to cover the first insulating layer andthe electrode. The first electrode of a light emitting element iselectrically connected to the electrode through a contact hole formed onthe second insulating layer. The light emitting element is formed byinterposing a light emitting layer between the first electrode and asecond electrode. The light emitting device further includes a filmformed of a different material from the second insulating layer onlybetween a top surface of the second insulating layer and the firstelectrode of a light emitting element and the wiring.

Another light emitting device of the invention includes a thin filmtransistor, a first insulating layer covering the thin film transistor,and an electrode which is formed over the first insulating layer and iselectrically connected to the thin film transistor through a contacthole formed on the first insulating layer. A wiring and a firstelectrode of a light emitting element are formed over a secondinsulating layer which is formed to cover the first insulating layer andthe electrode. The first electrode of a light emitting element iselectrically connected to the wiring through a contact hole formed onthe second insulating layer. The light emitting element is formed byinterposing a light emitting layer between the first electrode and asecond electrode. The light emitting device further includes a filmformed of a different material from the second insulating layer onlybetween a top surface of the second insulating layer and the wiring.

Another light emitting device of the invention includes a thin filmtransistor, a first insulating layer covering the thin film transistor,and an electrode which is formed over the first insulating layer and iselectrically connected to the thin film transistor through a contacthole formed on the first insulating layer. A wiring and a firstelectrode of a light emitting element are formed over a secondinsulating layer which is formed to cover the first insulating layer andthe electrode. The first electrode of a light emitting element iselectrically connected to the wiring through a contact hole formed onthe second insulating layer. The light emitting element is formed byinterposing a light emitting layer between the first electrode and asecond electrode. The light emitting device further includes a filmformed of a different material from the second insulating layer onlybetween a top surface of the first insulating layer and the electrode.

Another light emitting device of the invention includes a thin filmtransistor, a first insulating layer covering the thin film transistor,and an electrode which is formed over the first insulating layer and iselectrically connected to the thin film transistor through a contacthole formed on the first insulating layer. A wiring and a firstelectrode of a light emitting element are formed over a secondinsulating layer which is formed to cover the first insulating layer andthe electrode. The first electrode of a light emitting element iselectrically connected to the wiring through a contact hole formed onthe second insulating layer. The light emitting element is formed byinterposing a light emitting layer between the first electrode and asecond electrode. The light emitting device further includes a firstfilm formed of a different material from the first insulating layer onlybetween a top surface of the first insulating layer and the electrode,and a second film formed of a different material from the secondinsulating layer only between a top surface of the second insulatinglayer and the wiring and the first electrode of a light emittingelement.

Another light emitting device of the invention includes a thin filmtransistor, a first insulating layer covering the thin film transistor,and an electrode which is formed over the first insulating layer and iselectrically connected to the thin film transistor through a contacthole formed on the first insulating layer. A wiring and a firstelectrode of a light emitting element are formed over a secondinsulating layer which is formed to cover the first insulating layer andthe electrode. The first electrode of a light emitting element iselectrically connected to the wiring through a contact hole formed onthe second insulating layer. The light emitting element is formed byinterposing a light emitting layer between the first electrode and asecond electrode. The light emitting device further includes a firstfilm formed of a different material from the first insulating layer onlybetween a top surface of the first insulating layer and the electrode,and a second film formed of a different material from the secondinsulating layer only between a top surface of the second insulatinglayer and the wiring.

According to a method for manufacturing a light emitting device of theinvention, a thin film transistor is formed over an insulating surface,and an insulating layer is formed over the thin film transistor. Anetching stopper film is formed of a different material from theinsulating layer over the insulating layer, and an electrode whichpenetrates the etching stopper film and the insulating layer and iselectrically connected to the thin film transistor is formed.Subsequently, a first electrode of a light emitting element, which iselectrically in contact with a part of the electrode, is formed, and theetching stopper film is removed by using the electrode and the firstelectrode of a light emitting element as a mask.

According to another method for manufacturing a light emitting device ofthe invention, a thin film transistor is formed over an insulatingsurface, and an insulating layer is formed over the thin filmtransistor. An etching stopper film is formed of a different materialfrom the insulating layer over the insulating layer, and an electrodewhich penetrates the etching stopper film and the insulating layer andis electrically connected to the thin film transistor is formed.Subsequently, the etching stopper film is removed by using the electrodeas a mask, and a first electrode of a light emitting element, which iselectrically in contact with a part of the electrode, is formed.

According to another method for manufacturing a light emitting device ofthe invention, a thin film transistor is formed over an insulatingsurface formed over a substrate, and an insulating layer is formed overthe thin film transistor. An etching stopper film is formed of adifferent material from the insulating layer over the insulating layer,and an electrode which penetrates the etching stopper film and theinsulating layer and is electrically connected to the thin filmtransistor is formed. Subsequently, a first electrode of a lightemitting element, which is electrically in contact with a part of theelectrode, is formed, the etching stopper film is removed by using theelectrode and the first electrode of a light emitting element as a mask,and the substrate is heat-treated.

According to another method for manufacturing a light emitting device ofthe invention, a thin film transistor is formed over an insulatingsurface formed over a substrate, and an insulating layer is formed overthe thin film transistor. An etching stopper film is formed of adifferent material from the insulating layer over the insulating layer,and an electrode which penetrates the etching stopper film and theinsulating layer and is electrically connected to the thin filmtransistor is formed. Subsequently, the etching stopper film is removedby using the electrode as a mask, a first electrode of a light emittingelement, which is electrically in contact with a part of the electrode,is formed, and the substrate is heat-treated.

According to another method for manufacturing a light emitting device ofthe invention, a thin film transistor is formed over an insulatingsurface formed over a substrate, and an insulating layer is formed overthe thin film transistor. An etching stopper film is formed of adifferent material from the insulating layer over the insulating layer,and an electrode which penetrates the etching stopper film and theinsulating layer and is electrically connected to the thin filmtransistor is formed. Subsequently, a first electrode of a lightemitting element, which is electrically in contact with a part of theelectrode, is formed, the etching stopper film is removed by using theelectrode and the first electrode of a light emitting element as a mask,and the substrate is heat-treated in a vacuum.

According to another method for manufacturing a light emitting device ofthe invention, a thin film transistor is formed over an insulatingsurface formed over a substrate, and an insulating layer is formed overthe thin film transistor. An etching stopper film is formed of adifferent material from the insulating layer over the insulating layer,and an electrode which penetrates the etching stopper film and theinsulating layer and is electrically connected to the thin filmtransistor is formed. Subsequently, the etching stopper film is removedby using the electrode as a mask, a first electrode of a light emittingelement, which is electrically in contact with a part of the electrode,is formed, and the substrate is heat-treated in a vacuum.

According to another method for manufacturing a light emitting device ofthe invention, a thin film transistor is formed over an insulatingsurface formed over a substrate, and an insulating layer is formed overthe thin film transistor. An etching stopper film is formed of adifferent material from the insulating layer over the insulating layer,and an electrode which penetrates the etching stopper film and theinsulating layer and is electrically connected to the thin filmtransistor is formed. Subsequently, a first electrode of a lightemitting element, which is electrically in contact with a part of theelectrode, is formed, and the etching stopper film is removed by usingthe electrode and the first electrode of a light emitting element as amask. Then, a bank is formed to cover an edge portion of the firstelectrode of a light emitting element, the substrate is heat-treated ina vacuum, a light emitting film is continuously formed over the bank tothe first electrode, and a second electrode is formed to cover the bankand the light emitting film.

According to another method for manufacturing a light emitting device ofthe invention, a thin film transistor is formed over an insulatingsurface formed over a substrate, and an insulating layer is formed overthe thin film transistor. An etching stopper film is formed of adifferent material from the insulating layer over the insulating layer,and an electrode which penetrates the etching stopper film and theinsulating layer and is electrically connected to the thin filmtransistor is formed. Subsequently, the etching stopper film is removedby using the electrode as a mask, and a first electrode of a lightemitting element, which is electrically in contact with a part of theelectrode, is formed. Then, a bank is formed to cover an edge portion ofthe first electrode of a light emitting element, the substrate isheat-treated in a vacuum, a light emitting film is continuously formedover the bank to the first electrode of a light emitting element, and asecond electrode is formed to cover the bank and the light emittingfilm.

In a light emitting device having a structure of the invention, theamount of water remaining inside the light emitting device is reduced.Further, the deterioration of a light emitting element due to waterremaining inside can be suppressed.

In a light emitting device manufactured by a method for manufacturing alight emitting element of the present invention, the amount of waterremaining inside the light emitting device is reduced. Further, thedeterioration of a light emitting element due to water remaining insidecan be suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are cross-sectional views showing a light emittingdevice of the invention.

FIGS. 2A to 2C are cross-sectional views showing a light emitting deviceof the invention.

FIGS. 3A to 3D are cross-sectional views showing a method formanufacturing a light emitting device of the invention.

FIGS. 4A to 4D are cross-sectional views showing a method formanufacturing a light emitting device of the invention.

FIGS. 5A to 5D are cross-sectional views showing a method formanufacturing a light emitting device of the invention.

FIG. 6 is a top view showing an example of a module structure of theinvention.

FIGS. 7A to 7E show examples of an electronic device of the invention.

FIGS. 8A and 8B show examples of a structure of a light emitting deviceof the invention.

FIGS. 9A to 9F show examples of a pixel circuit.

FIG. 10 shows an example of a protective circuit.

FIGS. 11A and 11B are cross-sectional views showing a light emittingdevice of the invention.

FIGS. 12A and 12B are pictures showing an upper surface of a lightemitting device of the invention.

FIGS. 13A and 13B are pictures showing an upper surface of a lightemitting device of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a preferred embodiment mode of the present invention willbe described with reference to the attached drawings. The invention isnot limited to the following description. As is easily known to a personskilled in the art, the mode and the detail of the invention can bevariously changed without departing from the purpose and the scope ofthe invention. Thus, the invention is not interpreted while limiting tothe following description of the embodiment mode. Note that a referencenumeral may not be given to a portion having similar function and shape,and the description thereof may be omitted.

(Embodiment Mode 1)

FIGS. 1A and 1B are cross-sectional views showing part of a lightemitting device of the invention. A light emitting device of theinvention includes: a base insulating layer 101 formed over a substrate100; a thin film transistor 105 having a semiconductor layer 102 formedover the base insulating layer 101 as an active layer, a gate insulatinglayer 103, and a gate electrode 104; and a light emitting element 106.The thin film transistor 105 is covered with an insulating layer 107.Over the insulating layer 107, electrodes 108 and 109 of the thin filmtransistor 105 which electrically connect to the semiconductor layer 102through contact holes penetrating the insulating layer 107 and the gateinsulating layer 103, and a wiring (not shown) are formed. Note that thewiring may be formed at the same time as the electrodes 108 and 109, butmay be separately formed. A first electrode 110 of the light emittingelement 106 is formed to partially overlap and electrically connect tothe electrode 109 of the thin film transistor 105. An edge portion ofthe first electrode 110 is covered with a bank 113. The light emittingelement 106 is formed by interposing a light emitting layer 112, whichis continuously formed over the bank 113 to the first electrode 110,between the first electrode 110 and a second electrode 111. Light can beemitted from a portion of the light emitting layer 112, which isdirectly sandwiched between the first electrode 110 and the secondelectrode 111.

The insulating layer 107 is preferably formed of a material which has aself-planarizing property and can reduce irregularity caused by the thinfilm transistor 105 thereunder, in order to improve an aperture ratio.For example, a self-planarizing application film of acrylic, polyimide,siloxane (a material which is composed of a skeleton formed by the bondof silicon and oxygen and which includes either an organic groupcontaining at least hydrogen (such as an alkyl group or aromatichydrocarbon) or a fluoro group or both as a substituent), or the like ispreferably used.

In FIG. 1A, an etching stopper film 114 remains only between theelectrodes 108, 109, and 110 and the wiring, which are formed over theinsulating layer 107, and the insulating layer 107. In other words, itremains only between the electrodes 108 and 109 of the thin filmtransistor 105, the first electrode 110 of the light emitting element106, and the wiring (not shown) formed between the insulating layer 107and the bank 113, and a top surface of the insulating layer 107. Whenthe electrodes 108, 109, and 110 and the wiring formed between theinsulating layer 107 and the bank 113 are formed by etching, the etchingstopper film 114 has a function of preventing the insulating layer 107from being etched. In the case of the light emitting device of theinvention in this embodiment mode, the etching stopper film 114 isformed to cover the insulating layer 107; a contact hole is formed; theelectrodes 108, 109, and 110 and the wiring formed between theinsulating layer 107 and the bank 113 are formed thereover; and then, anexposed portion of the etching stopper film is removed. Thus, the lightemitting device of the invention can be formed to have such a structurethat the etching stopper film 114 remains only below the electrodes 108and 109, the wiring formed between the insulating layer 107 and the bank113, and the first electrode 110 of the light emitting element 106.

The etching stopper film 114 is formed of a water impermeable materialwhich is different from the insulating layer 107, with or withoutconductivity. A film which mainly contains silicon nitride is preferablyused as the etching stopper film. A material, having a sufficientlylower etching rate than those of the electrodes 108 and 109 at the timeof etching to form the electrodes 108 and 109, is used.

In the light emitting device of the invention having the structure, theetching stopper film 114 is not entirely formed over the insulatinglayer 107 at the time of performing heat treatment for removing water inthe insulating layer 107. Therefore, water is effectively removed from aportion where the etching stopper film 114 is removed, and the amount ofwater remaining in the insulating layer 107 can be reduced.

Since the etching stopper film 114 is not entirely formed over theinsulating layer 107, even when a small amount of water remaining in theinsulating layer 107 seeps into the layer where the light emittingelement 106 is formed, the water is not locally concentrated. Thus, aninfluence of water seeping from the insulating layer 107 on the lightemitting element 106 can be reduced. If the etching stopper film isentirely formed over the insulating layer 107, the water remaining inthe insulating layer 107 seeps, in a concentrated manner, from a tinypinhole in the etching stopper film or at the boundary with theelectrodes 108 and 109. Therefore, deterioration of the light emittingelement located in the periphery thereof is promoted.

Since the deterioration of the light emitting element 106 due to watercan be suppressed, improvement in display quality and reliability can berealized.

FIG. 1B is different from FIG. 1A in the way that there is an etchingstopper film 115 only below electrodes 108 and 109 of a thin filmtransistor 105 and a wiring (not shown) formed between an insulatinglayer 107 and a bank 113, but not below a first electrode 110 of a lightemitting element 106. Only the timing of removing the etching stopperfilm 115 is different, but an effect and a structure of the etchingstopper film are similar to that in FIG. 1A. In other words, the stepsare as follows: the insulating layer 107 and the etching stopper film115 are formed; the electrodes 108 and 109 of the thin film transistorand the wiring are formed; and an exposed portion of the etching stopperfilm 115 is removed before forming the first electrode 110 of the lightemitting element 106.

In order to completely remove a portion of the etching stopper film 115which is to be removed, it is effective to perform overetching to someextent. In this case, the insulating layer may be etched in some cases,depending on an etching rate of the insulating layer under removingconditions of the etching stopper film. For example, schematic diagramsof the structures corresponding to FIGS. 1A and 1B in the case ofperforming overetching are shown in FIGS. 11A and 11B, respectively. Asshown, a portion of the insulating layer 107 where the etching stopperfilm 115 remains is thicker than a portion of the insulating layer 107where the etching stopper film 115 is removed.

Note that overetching can be applied to other structures of theinvention. In that case, a portion of the insulating layer where theetching stopper remains is thicker than a portion of the insulatinglayer where the etching stopper film is removed.

FIG. 2A is a cross-sectional view of a light emitting device of theinvention, in which an electrode of a thin film transistor is formed ina different layer from a first electrode of a light emitting element.The light emitting device of the invention includes: a base insulatinglayer 301 formed over a substrate 300; a thin film transistor 315 havinga semiconductor layer 302 formed over the base insulating layer 301 asan active layer, a gate insulating layer 303, and a gate electrode 304;and a light emitting element 316. The thin film transistor 315 iscovered with a first insulating layer 305. Over the first insulatinglayer 305, a wiring (not shown) and electrodes 306 and 307 of the thinfilm transistor 315 which electrically connect to the semiconductorlayer 302 through contact holes penetrating the first insulating layer305 and the gate insulating layer 303 are formed. Note that the wiringmay be formed at the same time as the electrodes 306 and 307, or may beseparately formed. The wiring (not shown), the electrodes 306 and 307 ofthe thin film transistor 315, and the first insulating layer 305 arecovered with a second insulating layer 309. Over the second insulatinglayer 309, a first electrode 311 of the light emitting element 316,which electrically connects to the electrode 307 of the thin filmtransistor 315 through a contact hole penetrating the second insulatinglayer 309, is formed. The light emitting element 316 is formed byinterposing a light emitting layer 313 between the first electrode 311and a second electrode 314 which are formed over the second insulatinglayer 309. A wiring 310 may be formed over the second insulating layer309. When the second electrode 314 has high resistance, the wiring 310can be used as an auxiliary wiring by forming a contact hole reachingthe wiring 310 in a bank 312 and connecting the second electrode 314 tothe wiring 310. Naturally, the wiring 310 may be used for otherapplications as well as an auxiliary wiring.

In FIG. 2A, either the first insulating layer 305 or the secondinsulating layer 309 is formed of a material which has aself-planarizing property and can reduce irregularity caused by the thinfilm transistor 315 thereunder or the like, in order to improve anaperture ratio. For example, a self-planarizing application film ofacrylic, polyimide, siloxane, or the like is used.

An etching stopper film 308 is formed between a top surface of the firstinsulating layer 305 and the electrodes 306 and 307 of the thin filmtransistor 315 and the wiring (not shown) formed between the firstinsulating layer 305 and the bank 312. When the electrodes 306 and 307and the wiring formed between the first insulating layer 305 and thebank 312 are formed by etching, the etching stopper film 308 has afunction of preventing the first insulating layer 305 from being etched.In the case of the light emitting device of the invention in thisembodiment mode, the etching stopper film is formed to cover the firstinsulating layer 305; a conductive film is formed thereover, patterned,and etched to form the electrodes 306 and 307 and the wiring; and anexposed portion of the etching stopper film is removed. Thus, the lightemitting device of the invention can be formed to have such a structurethat the etching stopper film 308 remains only below the electrodes 306and 307 and the wiring formed between the first insulating layer 305 andthe bank 312.

The etching stopper film 308 is formed of a water impermeable materialwhich is different from the first insulating layer 305, with or withoutconductivity. A film which mainly contains silicon nitride is preferablyused as the etching stopper film. A material, having a sufficientlylower etching rate than those of the electrodes 306 and 307 and thewiring formed between the first insulating layer 305 and the bank 312 atthe time of etching to form the electrodes 306 and 307 and the wiring,is used.

An etching stopper film 317 is also formed between a top surface of thesecond insulating layer 309 and the first electrode 311 of the lightemitting element 316 and the wiring 310. When the first electrode 311and the wiring 310 are formed by etching, the etching stopper film 317has a function of preventing the second insulating layer 309 from beingetched. The etching stopper film 317 is formed as in the case with theetching stopper film 308: an etching stopper film is formed to cover thesecond insulating layer 309, the first electrode 311 and the wiring 310are formed thereover, and an exposed portion of the etching stopper filmis removed. Thus, the light emitting device of the invention can beformed to have such a structure that the etching stopper film 317remains only below the first electrode 311 and the wiring 310.

The etching stopper film 317 is formed of a material different from thesecond insulating layer 309, with or without conductivity. A film whichmainly contains silicon nitride is preferably used as the etchingstopper film. A material, having a sufficiently lower etching rate thanthose of the first electrode 311 and the wiring 310 at the time ofetching to form the first electrode 311 and the wiring 310, is used.

When either the etching stopper film 308 or the etching stopper film 317is not required in terms of the etching rate, the etching stopper filmmay not be formed.

In the light emitting device of the invention having the structure, theetching stopper films are not entirely formed over the first insulatinglayer 305 and the second insulating layer 309 at the time of performingheat treatment for removing water in the insulating layers. Therefore,water is effectively removed from a portion where the etching stopperfilms are removed, and the amount of water remaining in the firstinsulating layer 305 and the second insulating layer 309 can be reduced.

Since the etching stopper film 317 is not entirely formed over theinsulating layer, even when a small amount of water remaining in theinsulating layer seeps into the layer where the light emitting element316 is formed, the water is not locally concentrated. Thus, an influenceof water seeping from the second insulating layer 309 on the lightemitting element 316 can be reduced.

Since the deterioration of the light emitting element 316 due to watercan be suppressed, improvement in display quality and reliability can berealized.

FIG. 2B has a structure almost similar to FIG. 2A, but it shows anexample that a first insulating layer 350 is formed of an inorganic filmof silicon oxide, silicon nitride, or the like. Since such a film oftenhas a sufficient difference in an etching rate with electrodes 351 and352 of a thin film transistor 353 and a wiring, an etching stopper filmbetween the electrodes 351 and 352 and the wiring formed between a firstinsulating layer 350 and a second insulating layer 309 and a top surfaceof the first insulating layer 350 may not be formed. In that case, thefirst insulating layer 350 directly reflects irregularity thereunder;therefore, the second insulating layer 309 is formed of aself-planarizing material. For example, a self-planarizing applicationfilm of acrylic, polyimide, siloxane, or the like corresponds to this.Note that other structures and effects are similar to those in FIG. 2A,so description is omitted.

FIG. 2C has a structure almost similar to FIG. 2A, but the wiring 310 inFIG. 2A is used not as an auxiliary wiring for reducing apparentresistance of a second electrode of a light emitting element but as awiring 370 electrically connecting to the electrode 306 of the thin filmtransistor 315. The first electrode 311 of the light emitting element isconnected to the electrode 307 of the thin film transistor 315 notdirectly but through a wiring 371. Note that other structures andeffects are similar to those in FIG. 2A, so description is omitted.

In this embodiment mode, the structure that an etching stopper is formedbetween an electrode of a transistor and an insulating layer thereunder,the structure that an etching stopper is formed between a wiring and aninsulating layer thereunder, the structure that an etching stopper filmis formed between an electrode of a light emitting element and aninsulating layer thereunder, and the structure that an etching stopperis not formed between an electrode of a light emitting element and aninsulating layer thereunder are disclosed, and patterns that each of thestructures is applied to the structure that an electrode of a transistorand an electrode of a light emitting element are formed in the samelayer, the structure that they are formed in different layers, and thelike are described. However, the elements can be freely combined witheach other.

In this embodiment mode, the base insulating layer, the insulatinglayer, the first insulating layer, and the second insulating layer aredescribed as single layers; however, they may have multilayer structuresof two or more layers. In addition, the etching stopper film may also bea single layer or have a multilayer structure.

(Embodiment Mode 2)

A method for manufacturing a light emitting element of the invention isdescribed in this embodiment mode with reference to FIGS. 3A to 3D andFIGS. 4A to 4D.

An insulating layer 801 is formed over a substrate 800, and asemiconductor layer is then formed over the insulating layer 801 (FIG.3A).

Light transmitting glass, quartz, plastic (such as polyimide, acrylic,polyethylene terephthalate, polycarbonate, polyacrylate, orpolyethersulfone), or the like can be used as a material of thesubstrate 800. The substrate thereof may be used after being polished byCMP or the like, if necessary. In this embodiment mode, a glasssubstrate is used.

The insulating layer 801 is formed in order to prevent an elementexerting an adverse influence on characteristics of the semiconductorlayer, such as an alkali metal or an alkaline earth metal contained inthe substrate 800, from diffusing into the semiconductor layer. Siliconoxide, silicon nitride, silicon oxide containing nitrogen, siliconnitride containing oxygen, or the like can be used as a materialthereof, and the insulating layer is formed to be a single layer or tohave a laminated structure. Note that the insulating layer 801 is notnecessarily required to be formed when diffusion of an alkali metal oran alkaline earth metal need not be worried about.

The subsequently formed semiconductor layer is obtained by performinglaser crystallization on an amorphous silicon film in this embodimentmode. An amorphous silicon film is formed to be 25 nm to 100 nm(preferably, 30 nm to 60 nm) in thickness over the insulating layer 801.A known method such as a sputtering method, a low pressure CVD method,or a plasma CVD method can be used as a manufacturing method thereof.Subsequently, the amorphous silicon film is heat-treated at atemperature of 500° C. for one hour for dehydrogenation.

Then, the amorphous silicon film is crystallized with the use of a laserirradiation apparatus to form a crystalline silicon film. As to thelaser crystallization in this embodiment mode, an excimer laser is used,and an emitted laser beam is processed to have a linear beam spot withan optical system. The amorphous silicon film is irradiated therewith tobe a crystalline silicon film, and is used as the semiconductor layer.

As another method for crystallizing an amorphous silicon film, there isa crystallizing method only by heat treatment, a crystallizing method byheat treatment with the use of a catalytic element which promotescrystallization, or the like. Nickel, iron, palladium, tin, lead,cobalt, platinum, copper, gold, or the like can be used as the elementwhich promotes crystallization. By using the element, crystallizationcan be performed at a lower temperature in a shorter time, compared tothe case of performing crystallization only by heat treatment.Therefore, a glass substrate or the like is less damaged. In the case ofperforming crystallization only by heat treatment, a highly heatresistant quartz substrate or the like needs to be used as the substrate800.

Subsequently, addition of a very small amount of impurities, so-calledchannel doping, is performed on the semiconductor layer to control athreshold value, if necessary. An N type or P type impurity (phosphorus,boron, or the like) is added by an ion doping method to obtain arequired threshold value.

Thereafter, the semiconductor layer is patterned to have a predeterminedshape as shown in FIG. 3A, thereby obtaining an island-shapedsemiconductor layer 802. A photoresist is applied to the semiconductorlayer, exposed to light, and baked to form a resist mask having apredetermined shape over the semiconductor layer. Etching is performedusing the mask. Thus, the patterning is performed.

A gate insulating layer 803 is formed to cover the semiconductor layer802. The gate insulating layer 803 is formed of an insulating layercontaining silicon by a plasma CVD method or a sputtering method to be40 nm to 150 nm in thickness.

A gate electrode 804 is formed over the gate insulating layer 803. Thegate electrode 804 may be formed by using an element of Ta, W, Ti, Mo,Al, Cu, Cr, or Nd, or by using an alloy material or compound materialwhich mainly contains the element. A semiconductor film typified by apolycrystalline silicon film doped with an impurity element such asphosphorus may be used. Alternatively, an AgPdCu alloy may be used.

The gate electrode 804 is formed to be a single layer in this embodimentmode; however, it may be formed to have a laminated structure of two ormore layers (for example, a laminated structure of a tungsten layer as alower layer and a molybdenum layer as an upper layer). Theabove-mentioned material may be used, even in the case of forming a gateelectrode having a laminated structure. A combination thereof may alsobe appropriately selected.

The gate electrode 804 is processed by etching using a mask of aphotoresist.

The semiconductor layer 802 is added with a highly concentrated impuritywith the use of the gate electrode 804 as a mask. According to thisstep, a thin film transistor including the semiconductor layer 802, thegate insulating layer 803, and the gate electrode 804 is formed.

A manufacturing step of a thin film transistor is not particularlylimited, and it may be appropriately changed so that a transistor havinga desired structure can be formed.

A top gate thin film transistor using a crystalline silicon film whichis crystallized by employing laser crystallisation is used in thisembodiment mode; however, a bottom gate thin film transistor using anamorphous semiconductor film can be used for a pixel portion. Silicongermanium as well as silicon can be used for an amorphous semiconductor.In the case of using silicon germanium, the concentration of germaniumis preferably approximately 0.01 atomic % to 4.5 atomic %.

A microcrystalline semiconductor (semi-amorphous semiconductor) film, inwhich a crystal grain of 0 5 nm to 20 nm can be observed within anamorphous semiconductor, may be used. A microcrystalline state in whicha crystal grain of 0.5 nm to 20 nm can be observed is also referred toas a microcrystal (μc).

Semi-amorphous silicon (also referred to as SAS) that is asemi-amorphous semiconductor can be obtained by performing glowdischarge decomposition on a silicide gas. SiH₄ is used as a typicalsilicide gas. In addition, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or thelike can also be used as the silicide gas. The silicide gas may bediluted with hydrogen, or hydrogen and one or more rare gas elements ofhelium, argon, krypton, and neon, thereby making formation of the SASeasy. At this time, it is preferable to dilute the silicide gas so thata dilution ratio ranges from 10 times to 1000 times. Reaction productionof a film by glow discharge decomposition may be performed withpressures in the range of 0.1 Pa to 133 Pa. High-frequency powers of 1MHz to 120 MHz, preferably, 13 MHz to 60 MHz may be supplied to form aglow discharge. A substrate heating temperature is preferably 300° C. orless, and a recommended substrate heating temperature is in the range of100° C. to 250° C.

In the thus formed SAS, a Raman spectrum is shifted to a lower frequencyside than 520 cm⁻¹. A diffraction peak of (111) or (220) to be caused bya crystal lattice of silicon is observed in X-ray diffraction. The SAScontains hydrogen or halogen of at least 1 atomic % or more to terminatea dangling bond. It is desirable that an atmospheric constituentimpurity such as oxygen, nitrogen, or carbon is 1×10²⁰/cm³ or less as animpurity element in the film; specifically, an oxygen concentration is5×10¹⁹/cm³ or less, preferably 1×10¹⁹/cm³ or less. When the SAS isprocessed into a TFT, mobility thereof is as follows: μ=1 cm²/Vsec to 10cm²/Vsec. The SAS may be further crystallized with a laser.

Subsequently, an insulating layer 805 is formed to cover the gateelectrode 804 and the gate insulating layer 803. The insulating layer805 may be formed of acrylic, polyimide, or siloxane. In this embodimentmode, the insulating layer 805 is formed of siloxane (FIG. 3B).

An etching stopper film 806 is formed over the insulating layer 805. Theetching stopper film 806 is formed of a material different from theinsulating layer 805. Specifically, the etching stopper film is formedof a material having sufficient selectivity to the insulating layer 805so as not to etch the insulating layer 805, such as silicon nitride orsilicon nitride containing oxygen, when subsequently formed electrodes807 and 808 are formed by etching. Note that conductivity of a materialof the etching stopper film is not necessarily required. In thisembodiment mode, a silicon nitride film is formed as the etching stopperfilm 806 (FIG. 3C).

A contact hole penetrating the etching stopper film 806, the insulatinglayer 805, and the gate insulating layer 803 is formed to reach thesemiconductor layer 802 (FIG. 3D).

The contact hole may be formed and the insulating layer on the peripheryof the substrate may be removed using a resist by dry etching or wetetching. However, in some cases, the contact hole is preferably formedby etching plural times/multiple etching under different conditions,depending on a material of the insulating layer 805, the etching stopperfilm 806, and the gate insulating layer 803.

When wet etching is employed or the resist is removed by using aseparating solution, it is conceivable that water enters the insulatinglayer 805. Therefore, heat treatment may be performed in order to removewater in the insulating layer 805 after removing the resist. Heattreatment can be performed in any condition of atmospheric air, reducedpressure, a vacuum, and a specific gas atmosphere. The treatment may beperformed in a favorable condition. However, water cannot besufficiently removed at this time in some cases, since the insulatinglayer 805 is covered with the etching stopper film 806.

A conductive layer is formed to cover the contact hole, the insulatinglayer 805, and the etching stopper film 806. The conductive layer isprocessed to have a desired shape, thereby forming the electrodes 807and 808 and a wiring (not shown). They may be single layers of aluminum,copper, or the like; however, they are formed to have a laminatedstructure of molybdenum, aluminum, and molybdenum from a TFT side inthis embodiment mode. The conductive layer may also have anotherlaminated structure of titanium, aluminum, and titanium, or titanium,titanium nitride, aluminum, and titanium from a TFT side.

The conductive layer may be processed using a resist by dry etching orwet etching. The etching stopper film 806 can prevent the insulatinglayer 805 from being etched. As in the case of forming the contact hole,when wet etching is employed or the resist is removed by using aseparating solution, it is conceivable that water enters the insulatinglayer 805. Therefore, heat treatment may be performed to remove water inthe insulating layer 805 after removing the resist. Heat treatment canbe performed in any condition of atmospheric air, reduced pressure, avacuum, and a specific gas atmosphere. The treatment may be performed ina favorable condition. However, water cannot be sufficiently removed atthis time in some cases, since the insulating layer 805 is covered withthe etching stopper film 806.

According to the steps, a thin film transistor 809 in a pixel portion iscompleted.

After a light transmitting conductive layer is formed to partially coverthe electrode 808 of the thin film transistor 809 in the pixel portion,the light transmitting conductive layer is processed to form a firstelectrode 810 (FIG. 4A).

The light transmitting conductive layer may be processed using a resistby dry etching or wet etching. As in the case of forming the contacthole and the electrodes 807 and 808, when wet etching is employed or theresist is removed by using a separating solution, it is conceivable thatwater enters the insulating layer 805. Therefore, heat treatment may beperformed to remove water in the insulating layer 805 after removing theresist. Heat treatment can be performed in any condition of atmosphericair, reduced pressure, a vacuum, and a specific gas atmosphere. Thetreatment may be performed in a favorable condition. However, watercannot be sufficiently removed at this time in some cases, since theinsulating layer 805 is covered with the etching stopper film 806.

The first electrode 810 is electrically in contact with the electrode808 of the thin film transistor 809. The first electrode 810 may beformed of ITO (indium tin oxide), ITO containing silicon oxide, IZO(Indium Zinc Oxide) in which indium oxide contains zinc oxide of 2% to20%, zinc oxide, GZO (Gallium Zinc Oxide) in which zinc oxide containsgallium, or the like.

An exposed portion of the etching stopper film 806 is removed by etchingusing the electrodes 807 and 808 of the thin film transistor 809 and thefirst electrode 810 of a light emitting element as a mask. The etchingmay be performed by either wet etching or dry etching. In thisembodiment mode, dry etching is employed (FIG. 4B)

Note that the first electrode 810 and the etching stopper film 806 maybe simultaneously etched, when an etching method having high selectivitybetween the insulating layer 805 and the etching stopper film isemployed. Another method for removing the etching stopper film 806 bycontrolling the etching time to etch the etching stopper film 806 at thesame time as the etching of the electrodes 807 and 808 is conceivable.

When wet etching is employed, it is conceivable that water enters theinsulating layer 805. Therefore, heat treatment may be performed toremove water in the insulating layer 805 after removing the resist.Since the insulating layer 805 is not covered with the etching stopperfilm 806, water can be sufficiently removed. In the case where there isa step of exposing to water again in the following steps, heat treatmentmay be performed at a time later after the step. Heat treatment can beperformed in any condition of atmospheric air, reduced pressure, avacuum, and a specific gas atmosphere. The treatment may be performed ina favorable condition.

An insulating layer made of an organic material or an inorganic materialis formed to cover the etching stopper film 806 and the first electrode810. The insulating layer is processed to cover an edge portion of thefirst electrode and partially expose the first electrode, therebyforming a bank 811. The bank 811 is preferably formed of aphotosensitive organic material (acrylic, polyimide, or the like), butmay be formed of a non-photosensitive organic material or an inorganicmaterial. In this embodiment mode, photosensitive polyimide is used. Anend face of the bank 811, facing the first electrode, has curvature, andpreferably has a tapered shape in which the curvature continuouslychanges. Note that the bank 811 may be mixed with a black material suchas a pigment or carbon, and may be used as a black matrix (FIG. 4C).

The bank 811 may be processed by light-exposure and development when aphotosensitive material is used, or by dry etching or wet etching usinga resist when a non-photosensitive material is used. When development isperformed using a photosensitive material, wet etching is employed, orthe resist is removed by using a separating solution, it is conceivablethat water enters the bank 811 and the insulating layer 805. Therefore,heat treatment may be performed to remove water in the bank 811 and theinsulating layer 805 after removing the resist.

Heat treatment can be performed in any condition of atmospheric air,reduced pressure, a vacuum, and a specific gas atmosphere. The treatmentmay be performed in a favorable condition, but it is effective toperform heat treatment in a vacuum. Since the insulating layer 805 isnot entirely covered with the etching stopper film 806, water can besufficiently removed. In this embodiment mode, heat treatment isperformed in a vacuum.

A light emitting layer 812 is formed to cover an exposed portion of thefirst electrode 810 which is not covered with the bank 811. The lightemitting layer 812 may be formed by any of an evaporation method, anink-jet method, a spin coating method, and the like (FIG. 4D).

A second electrode 813 is formed to cover the light emitting layer 812.Thus, a light emitting element 814 including the first electrode 810,the light emitting layer 812, and the second electrode 813 can bemanufactured.

Note that it is preferable to form the light emitting layer 812 and thesecond electrode without exposing to atmospheric air after forming thebank 811 and heat treating in a vacuum. This is because atmosphericwater can be prevented from being introduced into the light emittingdevice.

A silicon oxide film containing nitrogen may be formed as a passivationfilm by a plasma CVD method. In the case of using a silicon oxide filmcontaining nitrogen, a silicon oxynitride film formed from SiH₄, N₂O,and NH₃, a silicon oxynitride film formed from SiH₄ and N₂O, or asilicon oxynitride film formed from a gas in which SiH₄ and N₂O arediluted with Ar may be formed by a plasma CVD method.

A silicon oxynitride hydride film formed from SiH₄, N₂O, and H₂ may beused as the passivation film. Naturally, the structure of thepassivation film is not limited to a single layer structure. Thepassivation film may have a single layer structure or laminatedstructure of another insulating layer containing silicon. In addition, amultilayer film of a carbon nitride film and a silicon nitride film, amultilayer film of styrene polymer, a silicon nitride film, or a diamondlike carbon film may be substituted for the silicon oxide filmcontaining nitrogen.

Then, a display portion is sealed. In the case of using an opposingsubstrate for sealing, the opposing substrate is attached by using aninsulating sealant so that an external connection portion is exposed. Adepression may be formed on the opposing substrate, and a drying agentmay be attached thereto. A space between the opposing substrate and asubstrate over which an element is formed may be filled with a dry inertgas such as nitrogen, or the opposing substrate may be formed byentirely applying a sealant to the pixel portion. It is preferable touse an ultraviolet curing resin or the like as the sealant. The sealantmay be mixed with a drying agent or particles for keeping a gapconstant. Then, the light emitting device is completed by attaching aflexible wiring board to the external connection portion.

Note that either an analog video signal or a digital video signal may beused for a light emitting display device of the invention having adisplay function. In the case of using a digital video signal, the videosignal can be divided into a video signal using voltage and a videosignal using current. A video signal, inputted to a pixel when a lightemitting element emits light, includes a constant voltage video signaland a constant current video signal. The constant voltage video signalincludes a signal in which voltage applied to a light emitting elementis constant and a signal in which current applied to a light emittingelement is constant. The constant current video signal includes a signalin which voltage applied to a light emitting element is constant and asignal in which current applied to a light emitting element is constant.Drive with the signal in which voltage applied to a light emittingelement is constant is constant voltage drive, and that with the signalin which current applied to a light emitting element is constant isconstant current drive. By constant current drive, constant current isapplied to a light emitting element, regardless of a change inresistance of the light emitting element. For a light emitting displaydevice of the invention and a driving method thereof, either a drivingmethod using voltage of a video signal or a driving method using currentof a video signal may be employed, and either constant voltage drive orconstant current drive may be employed.

Hereinabove, a method for manufacturing a light emitting device of theinvention is described.

(Embodiment Mode 3)

A method for manufacturing a light emitting device having the structureshown in FIG. 2B is described in this embodiment mode with reference toFIGS. 5A to 5D. Description up to formation of a base insulating layer801, a semiconductor layer 802, a gate insulating layer 803, and a gateelectrode 804 over a substrate 800 (FIG. 5A) is similar to thedescription of FIG. 3A in Embodiment Mode 2; therefore, the descriptionis omitted here.

After forming the gate electrode 804, a first insulating layer 850 isformed. The first insulating layer 850 can be formed of an organicinsulating layer of acrylic or polyimide, an inorganic insulating layermainly containing silicon oxide or silicon nitride, siloxane, or thelike. In this embodiment mode, silicon oxide is used for the firstinsulating layer 850. In the case of forming the first insulating layer850 by applying a self-planarizing material of acrylic, polyimide,siloxane, or the like, the light emitting device has such a structure asshown in FIG. 2A.

A contact hole is formed to penetrate the first insulating layer 850 andthe gate insulating layer 803. Then, a conductive layer is formed tocover the contact hole and the first insulating layer 850. Theconductive layer is processed to have a desired shape, thereby formingelectrodes 851 and 852 and a wiring (not shown). They may be singlelayers of aluminum, copper, or the like; however, they may be formed tohave laminated structures. As a laminated wiring, the followinglaminated structure can be employed: molybdenum, aluminum, andmolybdenum; titanium, aluminum, and titanium; titanium, titaniumnitride, aluminum, and titanium; or the like, from a semiconductor layerside.

Since silicon oxide is used for the first insulating layer 850 in thisembodiment mode, an etching stopper film need not be formed. However, inthe case of using, for the first insulating layer 850, a materialwithout a sufficient difference in an etching rate at the time ofetching the electrodes 851 and 852 and the wiring (not shown), anetching stopper film needs to be formed. In that case, after forming theelectrodes 851 and 852 and the wiring, an exposed portion of the etchingstopper film is preferably removed using the electrodes and the wiringas a mask.

The conductive layer may be processed and the contact hole may be formedby dry etching or wet etching using a resist. When wet etching isemployed or the resist is removed by using a separating solution, theinsulating layer is exposed to water. Therefore, heat treatment may beperformed after removing the resist. Heat treatment can be performed inany condition of atmospheric air, reduced pressure, a vacuum, and aspecific gas atmosphere. The treatment may be performed in a favorablecondition. However, heat treatment for dehydration is not necessarilyrequired in some cases, when the first insulating layer 850 is highlywater impermeable.

According to the steps, a thin film transistor 853 in a pixel portion iscompleted (FIG. 5B)

Subsequently, a second insulating layer 854 is formed to cover theelectrodes 851 and 852, the wiring, and the first insulating layer 850.The second insulating layer 854 may be formed of a similar material tothe above-mentioned material of the first insulating layer 850. In thisembodiment mode, the first insulating layer 850 is formed of siliconoxide which does not have a self-planarizing property and directlyreflects irregularity thereunder. Therefore, the second insulating layeris formed of a self-planarizing material of acrylic, polyimide, orsiloxane. In this embodiment mode, siloxane is used for the secondinsulating layer 854 (FIG. 5C).

An etching stopper film 855 is formed over the second insulating layer854. The etching stopper film 855 is formed of a water impermeablematerial different from the second insulating layer 854. Specifically,the etching stopper film is formed of a material having high selectivityto a wiring 856 and a first electrode 857 of a light emitting element tobe formed later, such as silicon nitride or silicon nitride containingoxygen. Note that conductivity is not necessarily required for amaterial of the etching stopper film. In this embodiment mode, a siliconnitride film is formed as the etching stopper film 855.

After forming the etching stopper film 855, a conductive film is formedthereover and processed to form a wiring 856. The wiring 856 correspondsto the wiring 310 in FIG. 2A. Since the etching stopper film 855 isformed, the wiring 856 can be formed without largely etching a lowerfilm at the time of etching the conductive film (FIG. 5D).

A first electrode 857 of a light emitting element is formed. The firstelectrode 857 is electrically connected to the electrode 852 of the thinfilm transistor 853 through a contact hole formed in the secondinsulating layer 854. After forming a light transmitting conductivelayer, the first electrode 857 is formed by processing the conductivelayer. The first electrode 857 may be formed of ITO (indium tin oxide),ITO containing silicon oxide, IZO (Indium Zinc Oxide) in which indiumoxide contains zinc oxide of 2% to 20%, zinc oxide, GZO (Gallium ZincOxide) in which zinc oxide contains gallium, or the like.

The conductive layer may be processed by dry etching or wet etchingusing a resist. When wet etching is employed or the resist is removed byusing a separating solution, it is conceivable that water enters thesecond insulating layer 854. Therefore, heat treatment may be performedto remove water in the second insulating layer 854 after removing theresist. Heat treatment can be performed in any condition of atmosphericair, reduced pressure, a vacuum, and a specific gas atmosphere. Thetreatment may be performed in a favorable condition. However, watercannot be sufficiently removed in some cases, since the secondinsulating layer 854 is covered with the etching stopper film 855.

An exposed portion of the etching stopper film 855 is removed by etchingusing the wiring 856 and the first electrode 857 of the light emittingelement as a mask. The etching may be performed by either wet etching ordry etching. In this embodiment mode, dry etching is employed. Note thatthe first electrode 857 and the etching stopper film 855 may besimultaneously etched, in the case of employing an etching method havinghigher selectivity between the first electrode 857, and the secondinsulating layer 854 and the wiring 856. When the first electrode 857and the wiring 856 can have high selectivity to the second insulatinglayer 854, the etching stopper film need not be formed. The etchingstopper film may be used only at the time of forming either of them.

When wet etching is employed, it is conceivable that water enters thesecond insulating layer 854. Therefore, heat treatment may be performedto remove water in the second insulating layer 854 after removing theresist. Since the second insulating layer 854 is not covered with theetching stopper film 855, water can be sufficiently removed. In the casewhere there is a step of exposing to water again in the following steps,heat treatment may be performed at a time later after the step. Heattreatment can be performed in any condition of atmospheric air, reducedpressure, a vacuum, and a specific gas atmosphere. The treatment may beperformed in a favorable condition.

Subsequent steps are similar to those in FIG. 4C and later; therefore,description is omitted.

Hereinabove, a method for manufacturing a light emitting device of theinvention is described. Note that the method for manufacturing a lightemitting device in this embodiment mode can be appropriately combinedwith that in Embodiment Mode 2.

(Embodiment Mode 4)

The appearance of a panel of a light emitting device which correspondsto one mode of the invention is described in this embodiment mode withreference to FIG. 6. FIG. 6 is a top view of a panel in which atransistor and a light emitting element formed over a substrate aresealed with a sealant formed between the substrate and an opposingsubstrate 4006.

A sealant 4005 is provided to surround a pixel portion 4002, a signalprocessing circuit 4003, a signal line driver circuit 4020, and ascanning line driver circuit 4004 which are provided over a substrate4001. The opposing substrate 4006 is provided over the pixel portion4002, the signal processing circuit 4003, the signal line driver circuit4020, and the scanning line driver circuit 4004. Thus, the pixel portion4002, the signal processing circuit 4003, the signal line driver circuit4020, and the scanning line driver circuit 4004 are sealed with thesubstrate 4001, the sealant 4005, and the opposing substrate 4006,together with a filler.

The pixel portion 4002, the signal processing circuit 4003, the signalline driver circuit 4020, and the scanning line driver circuit 4004which are provided over the substrate 4001 have a plurality of thin filmtransistors.

A lead wiring corresponds to a wiring for supplying signals or powervoltage to the pixel portion 4002, the signal processing circuit 4003,the signal line driver circuit 4020, and the scanning line drivercircuit 4004. The lead wiring is connected to a connection terminal, andthe connection terminal is electrically connected to a terminal includedin a flexible printed circuit (FPC) 4018 through an anisotropicconductive film

An ultraviolet curing resin or a thermosetting resin as well as an inertgas such as nitrogen or argon can be used as the filler. Polyvinylchloride, acrylic, polyimide, an epoxy resin, a silicone resin,polyvinyl butyral, or ethylene vinylene acetate can be used.

A display device of the invention includes, in its category, a panelprovided with a pixel portion having a light emitting element and amodule in which an IC is mounted on the panel.

(Embodiment Mode 5)

Examples of an electronic device of the invention mounted with a module,one of whose example is described in Embodiment Mode 4, can be cited asfollows: a camera such as a video camera or a digital camera, a goggletype display (head mounted display), a navigation system, an audioreproducing device (a car audio component or the like), a computer, agame machine, a portable information terminal (a mobile computer, acellular phone, a portable game machine, an electronic book, or thelike), and an image reproducing device including a recording medium(specifically, a device capable of processing data in a recording mediumsuch as a Digital Versatile Disc (DVD) and having a display that candisplay the image of the data), and the like. Practical examples ofthese electronic devices are shown in FIGS. 7A to 7E.

FIG. 7A shows a light emitting display device. A television set, acomputer monitor, or the like is regarded as this. The light emittingdisplay device includes a chassis 2001, a display portion 2003, aspeaker portion 2004, and the like. In the light emitting display deviceaccording to the invention, the variation in an emission spectrumdepending on a viewing angle with respect to a side from whichluminescence is extracted in the display portion 2003 can be reduced,and display quality is improved. A pixel portion is preferably providedwith a polarizing plate or a circularly polarizing plate to enhancecontrast. For example, a quarter-wave plate, a half-wave plate, and apolarizing plate may be sequentially formed over a sealing substrate.Further, an anti-reflective film may be provided over the polarizingplate.

FIG. 7B shows a cellular phone, which includes a main body 2101, achassis 2102, a display portion 2103, an audio input portion 2104, anaudio output portion 2105, an operation key 2106, an antenna 2108, andthe like. In the cellular phone according to the invention, thedeterioration of a light emitting element in the display portion 2103 issuppressed, thereby improving reliability.

FIG. 7C shows a computer, which includes a main body 2201, a chassis2202, a display portion 2203, a keyboard 2204, an external connectionport 2205, a pointing mouse 2206, and the like. In the computeraccording to the invention, the variation in an emission spectrumdepending on a viewing angle with respect to a side from whichluminescence is extracted in the display portion 2203 can be reduced,and display quality is improved. Although a laptop computer is shown inFIG. 7C as an example, the invention can be applied to a desktopcomputer in which a hard disk and a display portion are integrated, andthe like.

FIG. 7D shows a mobile computer, which includes a main body 2301, adisplay portion 2302, a switch 2303, operation keys 2304, an infraredport 2305, and the like. In the mobile computer according to theinvention, the deterioration of a light emitting element in the displayportion 2302 is suppressed, thereby improving reliability.

FIG. 7E shows a portable gaming machine, which includes a chassis 2401,a display portion 2402, a speaker portion 2403, operation keys 2404, arecording medium insertion portion 2405, and the like. In the portablegaming machine according to the invention, the deterioration of a lightemitting element in the display portion 2402 is suppressed, therebyimproving reliability.

As described above, the applicable range of the invention is so widethat the invention can be applied to electronic devices of variousfields.

(Embodiment Mode 6)

A structure of a light emitting layer is described in detail in thisembodiment mode.

The light emitting layer may be made of a charge injection transportmaterial and a light emitting material including an organic compound orinorganic compound. The light emitting layer includes one or pluralkinds of layers of a low molecular weight organic compound, anintermediate molecular weight organic compound (referring to an organiccompound which does not have sublimation property and has the number ofmolecules of 20 or less or a molecular chain length of 10 μm or less),and a high molecular weight organic compound. The light emitting layermay be combined with an electron injection transport or hole injectiontransport inorganic compound.

As a highly electron transporting material among charge injectiontransport materials, a metal complex that has a quinoline skeleton or abenzoquinoline skeleton such as tris(8-quinolinolato)aluminum [Alq₃],tris(5-methyl-8-quinolinolato)aluminum [Almq₃],bis(10-hydroxybenzo[h]-quinolinato)beryllium [BeBq₂], orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum [BAlq], or thelike can be used. As a highly hole transporting material, an aromaticamine compound (that is, a compound having a benzene ring-nitrogen bond)such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl [α-NPD],4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl [TPD],4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine [TDATA], or4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]triphenylamine [MTDATA]can be used.

As a highly electron injecting material among charge injection transportmaterials, a compound of an alkali metal or an alkaline earth metal suchas lithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂) can be used. In addition, the highly electron injecting materialmay be a mixture of a highly electron transporting material such as Alq₃and an alkaline earth metal such as magnesium (Mg).

As a highly hole injecting material among charge injection transportmaterials, metal oxide such as molybdenum oxide (MoOx), vanadium oxide(VOx), ruthenium oxide (RuOx), tungsten oxide (WOx), or manganese oxide(MnOx) can be used. In addition, a phthalocyanine compound such asphthalocyanine [H₂Pc] or copper phthalocyanine (CuPC) can be used.

The light emitting layer may have a structure for performing colordisplay by providing each pixel with light emitting layers havingdifferent emission wavelength bands. Typically, a light emitting layercorresponding to each color of R (red), G (green), and B (blue) isformed. In this case, color purity can be increased and a pixel portioncan be prevented from having a mirror surface (glare) by providing alight emitting side of a pixel with a filter (colored layer) whichtransmits light of an emission wavelength band. Providing a filter(colored layer) can omit a circularly polarizing plate or the like whichis conventionally used to prevent a pixel portion from having a mirrorsurface (glare) and can eliminate the loss of light that light isreduced to half due to the use of a polarizing plate. Further, a changein hue, which occurs when a pixel portion (display screen) is obliquelyseen, can be reduced.

A light emitting material includes various materials. As a low molecularweight organic light emitting material,4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran[DCJT], 4-dicyanomethylene-2-t-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran [DCJTB], periflanthene,2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzene, N,N′-dimethyl quinacridon [DMQd],coumarin 6, coumarin 545T, tris(8-quinolinolate)aluminum [Alq₃],9,9′-bianthryl, 9,10-diphenylanthracene [DPA],9,10-bis(2-naphthyl)anthracene [DNA], or the like can be used. Inaddition, another material can also be used.

A high molecular weight organic light emitting material is physicallystronger than a low molecular weight material and is superior indurability of the element. In addition, a high molecular weight materialcan be used for application; therefore, the element is relatively easilymanufactured. A structure of a light emitting element using a highmolecular weight organic light emitting material is basically the sameas that of a light emitting element using a low molecular weight organiclight emitting material: a cathode, an organic light emitting layer, andan anode from the side of a semiconductor layer. However, it isdifficult to form such a laminated structure as in the case of using alow molecular weight organic light emitting material, when a lightemitting layer using a high molecular weight organic light emittingmaterial is formed. A two-layer structure is employed in many cases.Specifically, the light emitting element using a high molecular weightorganic light emitting material has a structure of a cathode, a lightemitting layer, a hole transport layer, and an anode from the side of asemiconductor layer.

The emission color is determined by the material of the light emittinglayer. Therefore, a light emitting element that emits desired light canbe formed by selecting an appropriate material of the light emittinglayer. As a high molecular weight light emitting material that can beused to form the light emitting layer, apolyparaphenylene-vinylene-based material, a polyparaphenylene-basedmaterial, a polythiophen-based material, or a polyfluorene-basedmaterial can be used.

As a polyparaphenylene-vinylene-based material, a derivative ofpoly(paraphenylene vinylene) [PPV], for example,poly(2,5-dialkoxy-1,4-phenylene vinylene) [RO-PPV],poly(2-(2′-ethyl-hexoxy)-5-metoxy-1,4-phenylene vinylene) [MEH-PPV],poly(2-(dialkoxyphenyl)-1,4-phenylene vinylene) [ROPh-PPV], or the likecan be used. As a polyparaphenylene-based material, a derivative ofpolyparaphenylene [PPP], for example, poly(2,5-dialkoxy-1,4-phenylene)[RO-PPP], poly(2,5-dihexoxy-1,4-phenylene), or the like can be used. Asa polythiophene-based material, a derivative of polythiophene [PT], forexample, poly(3-alkylthiophene) [PAT], poly(3-hexylthiophene) [PHT],poly(3-cyclohexylthiophene) [PCHT], poly(3-cyclohexyl-4-methylthiophene)[PCHMT], poly(3,4-dicyclohexylthiophene) [PDCHT],poly[3-(4-octylphenyl)-thiophene] [POPT], poly[3-(4-octylphenyl)-2,2bithiophene] [PTOPT], or the like can be used. As the polyfluorene-basedmaterial, a derivative of polyfluorene [PF], for example,poly(9,9-dialkylfluorene) [PDAF], poly(9,9-dioctylfluorene) [PDOF], orthe like can be used.

Note that a hole injection property from an anode can be enhanced byinterposing a high molecular weight organic light emitting materialhaving a hole transporting property between an anode and a highmolecular weight organic light emitting material. This hole transportingmaterial is generally dissolved into water together with an acceptormaterial, and the solution is applied by a spin coating method or thelike. Since the hole transporting material is insoluble in an organicsolvent, a laminate with the above-described organic light emittingmaterial can be formed. A mixture of PEDOT and camphor sulfonic acid(CSA) that serves as an acceptor material, a mixture of polyaniline[PANT] and polystyrene sulfonic acid [PSS] that serves as an acceptormaterial, and the like can be used as the hole transporting highmolecular weight organic light emitting material.

In addition, the light emitting layer can be formed to emit monochromeor white light. In the case of using a white light emitting material, afilter (colored layer) which transmits light having a specificwavelength is provided on a light emitting side of a pixel, therebyperforming color display.

In order to form a light emitting layer which emits white light, forexample, Alq₃, Alq₃ partially doped with Nile red that is a red lightemitting pigment, p-EtTAZ, and TPD (aromatic diamine) are sequentiallylaminated by an evaporation method to obtain white light. When the lightemitting layer is formed by an application method using spin coating,the layer after application is preferably baked by vacuum heating. Forexample, an aqueous solution of polyethylene dioxythiopheneypoly(styrenesulfonic acid) (PEDOT/PSS) may be entirely applied and baked to form alayer that functions as a hole injection layer. Then, a polyvinylcarbazole (PVK) solution doped with a light emitting center pigment(such as 1,1,4,4-tetraphenyl-1,3-butadiene (TPB),4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran (DCM1),Nile red, or coumarin 6) may be entirely applied and baked to form alayer that functions as a light emitting layer.

The light emitting layer may be formed to be a single layer. Forexample, a 1,3,4-oxadiazole derivative (PBD) having an electrontransporting property may be dispersed in polyvinyl carbazole (PVK)having a hole transporting property. Another method to obtain whitelight emission is to disperse PBD of 30 wt % and to disperse four kindsof pigments (TPB, coumarin 6, DCM1, and Nile red) in appropriateamounts. In addition to the light emitting elements described here thatprovide white light emission, a light emitting element that provides redlight emission, green light emission, or blue light emission can bemanufactured by appropriately selecting materials of the light emittinglayer.

Further, a triplet excitation light emitting material including a metalcomplex or the like as well as a singlet excitation light emittingmaterial may be used for the light emitting layer. For example, amongpixels emitting red, green, and blue light, a pixel emitting red lightwhose luminance is reduced by half in a relatively short time is made ofa triplet excitation light emitting material and the rest are made of asinglet excitation light emitting material. A triplet excitation lightemitting material has a characteristic that the material has a goodluminous efficiency and consumes less power to obtain the sameluminance. When a triplet excitation light emitting material is used fora red pixel, only small amount of current needs to be applied to a lightemitting element. Thus, reliability can be improved. A pixel emittingred light and a pixel emitting green light may be formed of a tripletexcitation light emitting material and a pixel emitting blue light maybe formed of a singlet excitation light emitting material to achieve lowpower consumption. Low power consumption can be further achieved byforming a light emitting element which emits green light that has highvisibility with a triplet excitation light emitting material.

A metal complex used as a dopant is an example of a triplet excitationlight emitting material, and a metal complex having platinum that is athird transition series element as a metal center, a metal complexhaving iridium as a metal center, and the like are known. A tripletexcitation light emitting material is not limited to the abovecompounds. A compound having the above described structure and anelement belonging to any of Groups 8 to 10 of the periodic table as ametal center can also be used.

The above described materials for forming the light emitting layer arejust examples. A light emitting element can be formed by appropriatelylaminating functional layers such as a hole injection transport layer, ahole transport layer, an electron injection transport layer, an electrontransport layer, a light emitting layer, an electron blocking layer, anda hole blocking layer. Further, a mixed layer or a mixed junction may beformed by combining these layers. The layer structure of the lightemitting layer can be varied. Instead of providing a specific electroninjection region or light emitting region, modification such asproviding an electrode for the purpose or providing a dispersed lightemitting material is acceptable as long as it does not deviate from thescope of the invention.

A light emitting element formed with the above described material emitslight by being biased in a forward direction. A pixel of a displaydevice formed with a light emitting element can be driven by a simplematrix mode or an active matrix mode. In either mode, each pixel emitslight by applying a forward bias thereto in specific timing; however,the pixel is in a non-light-emitting state for a certain period.Reliability of a light emitting element can be improved by applying areverse bias at this non-light-emitting time. In a light emittingelement, there is a deterioration mode in which emission intensity isdecreased under specific driving conditions or a deterioration mode inwhich a non-light-emitting region is enlarged in the pixel and luminanceis apparently decreased. However, progression of deterioration can beslowed down by alternating driving of applying a forward bias and areverse bias. Thus, the reliability of a light emitting device can beimproved.

(Embodiment Mode 7)

An example of a light emitting device using the invention is describedin this embodiment mode with reference to FIGS. 8A and 8B. In thisembodiment mode, a thin film transistor 809 having an LDD structure isconnected to a light emitting element 814 through an electrode 808 ofthe thin film transistor.

FIG. 8A shows a structure in which a first electrode 810 is formed of alight transmitting conductive film and light emitted from a lightemitting layer 812 is extracted to the side of a substrate 800. Notethat reference numeral 815 denotes an opposing substrate and is fixed tothe substrate 800 with the use of a sealant or the like after the lightemitting element 814 is formed. A space between the opposing substrate815 and the element is filled with a light transmitting resin 816 or thelike, and sealing is performed. Accordingly, the deterioration of thelight emitting element 814 due to moisture can be further suppressed.The resin 816 is preferably hygroscopic. When a highly lighttransmitting drying agent is dispersed in the resin 816, an influence ofthe moisture can be further reduced. Therefore, it is a more preferablemode.

FIG. 8B shows a structure in which both a first electrode 810 and asecond electrode 813 are formed of a light transmitting conductive filmand light can be emitted to both sides of a substrate 800 and anopposing substrate 815. In this structure, a screen can be preventedfrom being transparent by providing a polarizing plate 817 outside thesubstrate 800 and the opposing substrate 815, and visibility isincreased. A protective film 818 is preferably provided outside thepolarizing plate 817.

(Embodiment Mode 8)

A pixel circuit, a protective circuit, and operation thereof aredescribed in this embodiment mode.

In a pixel shown in FIG. 9A, a signal line 1410 and power supply lines1411 and 1412 are arranged in a column direction and a scanning line1414 is arranged in a row direction. In addition, the pixel includes aswitching TFT 1401, a driving TFT 1403, a current control TFT 1404, acapacitor element 1402, and a light emitting element 1405.

A pixel shown in FIG. 9C is different in the way that a gate electrodeof a TFT 1403 is connected to a power supply line 1412 arranged in a rowdirection, but other than that, the pixel has a similar structure tothat of the pixel shown in FIG. 9A. In other words, equivalent circuitdiagrams of both of the pixels shown in FIGS. 9A and 9C are the same.However, each power supply line is formed using a conductive layer in adifferent layer when the power supply line 1412 is arranged in a columndirection (FIG. 9A) and when the power supply line 1412 is arranged in arow direction (FIG. 9C). Here, a wiring connected to the gate electrodeof the driving TFT 1403 is focused and the figures are separately shownin FIGS. 9A and 9C to show that the wirings are formed in differentlayers.

In the pixels shown in FIG. 9A and FIG. 9C, the TFTs 1403 and 1404 areconnected in series. A channel length L(1403) and a channel widthW(1403) of the TFT 1403 and a channel length L(1404) and a channel widthW(1404) of the TFT 1404 are preferably set to satisfyL(1403)/W(1403):L(1404)/W(1404)=5 to 6000:1.

Note that the TFT 1403 operates in a saturation region and has a role ofcontrolling the amount of electric current flowing through the lightemitting element 1405, and the TFT 1404 operates in a linear region andhas a role of controlling supply of electric current to the lightemitting element 1405. It is preferable, from the viewpoint of themanufacturing steps, that the TFTs have the same conductivity. In thisembodiment mode, the TFTs are formed to be n-channel TFTs. Further, theTFT 1403 may be a depletion mode TFT as well as an enhancement mode TFT.In the present invention having the above structure, the TFT 1404operates in a linear region, so that slight variation in gate-sourcevoltage (Vgs) of the TFT 1404 does not affect the amount of electriccurrent of the light emitting element 1405. In other words, the amountof electric current of the light emitting element 1405 is determined bythe TFT 1403 which operates in a saturation region. According to theabove-described structure, luminance variation of the light emittingelement, which is caused by the variation in characteristics of the TFT,can be improved, and a display device with improved image quality can beprovided.

In pixels shown in FIGS. 9A to 9D, the TFT 1401 controls the input of avideo signal to the pixel. When the TFT 1401 is turned on, the videosignal is inputted to the pixel. Then, voltage of the video signal isstored in the capacitor element 1402. FIGS. 9A and 9C each show astructure in which the capacitor element 1402 is provided; however, theinvention is not limited thereto. When a gate capacitor or the like canbe used as the capacitor that can hold a video signal, the capacitorelement 1402 may not be provided.

The pixel shown in FIG. 9B has the same structure as that of the pixelshown in FIG. 9A except that a TFT 1406 and a scanning line 1415 areadded. In the same manner, the pixel shown in FIG. 9D has the samestructure as that of the pixel shown in FIG. 9C except that a TFT 1406and a scanning line 1415 are added.

In the TFT 1406, “on” or “off” is controlled by the scanning line 1415that is newly arranged. When the TFT 1406 is turned on, an electriccharge held in the capacitor element 1402 is discharged, and the TFT1404 is then turned off. In other words, it is possible to make a statein which current is forced not to flow through the light emittingelement 1405 by arranging the TFT 1406. Therefore, the TFT 1406 can bereferred to as an erasing TFT. Accordingly, in the structures in FIGS.9B and 9D, a lighting period can be started simultaneously with orimmediately after the start of a writing period without waiting for thewriting of signals in all pixels. Consequently, a duty ratio can beimproved.

In a pixel shown in FIG. 9E, a signal line 1410 and a power supply line1411 are arranged in a column direction, and a scanning line 1414 isarranged in a row direction. In addition, the pixel includes a switchingTFT 1401, a driving TFT 1403, a capacitor element 1402, and a lightemitting element 1405. A pixel shown in FIG. 9F has the same structureas that of the pixel shown in FIG. 9E except that a TFT 1406 and ascanning line 1415 are added. A duty ratio can be increased by arrangingthe TFT 1406 also in the structure of FIG. 9F.

As described above, various pixel circuits can be adopted. It ispreferable to make a semiconductor film of a driving TFT largespecifically in the case of forming a thin film transistor with anamorphous semiconductor film or the like. Therefore, the pixel circuitis preferably a top emission type, which emits light from anelectroluminescent layer to the side of a sealing substrate.

Such an active matrix light emitting device is considered to beadvantageous to low voltage driving when a pixel density is increased,since each pixel is provided with TFTs.

In this embodiment mode, an active matrix light emitting device in whicheach pixel is provided with TFTs is described. However, a passive matrixlight emitting device in which every column is provided with TFTs can beformed. In the passive matrix light emitting device, TFTs are notprovided for each pixel; therefore, a high aperture ratio can beobtained. In the case of a light emitting device which emits light tothe both sides of an electroluminescent layer, transmittance can beincreased by using the passive matrix display device.

The case of providing a scanning line and a signal line with a diode asa protective circuit is described with reference to an equivalentcircuit shown in FIG. 9E.

In FIG. 10, a pixel portion 1500 is provided with TFTs 1401 and 1403, acapacitor element 1402, and a light emitting element 1405. A signal line1410 is provided with diodes 1561 and 1562. The diodes 1561 and 1562 aremanufactured according to the above embodiment mode as in the case ofthe TFT 1401 or 1403 and include a gate electrode, a semiconductorlayer, a source electrode, a drain electrode, and the like. The diodes1561 and 1562 operate by connecting the gate electrode to the drainelectrode or the source electrode.

Common potential lines 1554 and 1555 connected to the diodes are formedin the same layer as the gate electrode. Therefore, a contact hole needsto be formed in a gate insulating layer to connect the gate electrode tothe source electrode or the drain electrode of the diode.

A diode provided for a scanning line 1414 has a similar structure. Ascanning line 1414 is provided with diodes 1563 and 1564.

Thus, a protective diode provided for an input stage can besimultaneously formed according to the invention. Note that a positionwhere the protective diode is formed is not limited thereto, and theprotective diode can be provided between a driver circuit and a pixel.

[Embodiment 1]

A comparison of the number of regions where emission intensity wasdecreased was made, between a light emitting device to which theinvention was applied (Example 1) and a light emitting device to whichthe invention was not applied as a comparative example (ComparativeExample 1), after using the light emitting devices for a certain periodof time under given conditions. Note that a structure of the lightemitting device, Example 1, corresponds to the structure shown in FIG.1A. A structure of the light emitting device, Comparative Example 1,corresponds to a structure in which the etching stopper film 114 isentirely formed over the insulating layer 107 in FIG. 1A. The result ofcomparison is shown in Table 1. Note that the etching stopper film wasformed of a material which mainly contains silicon nitride in thisembodiment.

TABLE 1 number of regions where emission intensity is decreasedComparative Example 1 0 Example 1 9 *Temperature: 85° C., after 100hours

Table 1 shows the increased number of regions where emission intensitywas decreased, which was visually counted by examining a picture takenafter performing a storage test at the temperature of 85° C. for 100hours. The number of large-area regions across a plurality of pixels,where emission intensity was decreased was counted. As shown in Table 1,a region where emission intensity was decreased did not newly appear inthe light emitting device having the structure of the invention, evenafter 100 hours at 85° C. On the other hand, in Comparative Example 1,many regions where emission intensity was decreased newly appeared afterthe same period of time under the same conditions.

[Embodiment 2]

A comparison of the number of regions where emission intensity wasdecreased was made, between a light emitting device to which theinvention was applied (Example 2) and light emitting devices to whichthe invention was not applied as comparative examples (ComparativeExamples 2 to 5), after using the light emitting devices for a certainperiod of time under given conditions. Note that a structure of thelight emitting device, Example 2, corresponds to the structure shown inFIG. 1B. The structure was manufactured by controlling etching time toetch at the same time as etching of the electrodes 108 and 109 so thatthe etching stopper film 115 was removed.

Structures of the light emitting devices, Comparative Examples 2 to 5,correspond to a structure in which the etching stopper film 115 isentirely formed over the insulating layer 107 in FIG. 1B. A remainingfilm thickness means a thickness of the etching stopper film 115 exceptbetween the electrodes 108 and 109 and the insulating layer 107.Conditions were set by changing a thickness of the etching stopper film115 at the time of formation. The result of comparison is shown in Table2. Note that the etching stopper film was formed of a material whichmainly contains silicon nitride in this embodiment.

TABLE 2 number of pixels remaining film where emission thickness (nm)intensity is decreased Example 2 0 0 Comparative Example 2 30 84Comparative Example 3 60 104 Comparative Example 4 60 62 ComparativeExample 5 90 42 *Temperature: 85° C., after 80 hours

Table 2 shows the increased number of pixels where emission intensitywas decreased, which was visually counted after performing a storagetest at a temperature of 85° C. for 80 hours. Table 2 shows the numbercounted pixel by pixel, differently from Table 1. As shown in Table 2, apixel where emission intensity was decreased did not newly appear in thelight emitting device having a structure of the invention, even after 80hours at 85° C. On the other hand, in Comparative Examples 2 to 5, manypixels where emission intensity was decreased newly appeared after thesame period of time under the same conditions, although there weredifferences in the number.

FIGS. 12A and 12B are pictures of the light emitting device having astructure corresponding to FIG. 1B, similarly to Example 2, which aretaken with all pixels turned on. A brightly displaying portioncorresponds to a pixel. Note that a light emitting material which emitsgreen light is used for the light emitting device shown in FIGS. 12A and12B. FIGS. 12A and 12B show four corners, side portions between thecorners, and the center of a pixel portion in the light emitting device,but do not show an entire pixel portion.

FIG. 12A is a picture taken before performing a storage test at 85° C.Display is favorably performed without a pixel where emission intensityis decreased. FIG. 12B is a picture taken with all pixels turned onafter storage at 85° C. for 1024 hours. The favorable display ismaintained without generating a pixel where light intensity isdecreased, even after 1024 hours at 85° C.

FIGS. 13A and 13B are pictures of a light emitting device, ComparativeExample 6, in which the etching stopper film 115 is entirely formed overthe insulating layer 107 in FIG. 1B, which are taken with all pixelsturned on. Similarly to Example 2, a brightly displaying portioncorresponds to a pixel, and a light-emitting material which emits greenlight is used. FIGS. 13A and 13B show four corners, side portionsbetween the corners, and the center of a pixel portion in the lightemitting device, but do not show an entire pixel portion.

FIG. 13A is a picture taken before performing a storage test at 85° C.Regions where emission intensity is decreased exist in parts. However,initial failure and deterioration cannot be distinguished from eachother at this time. FIG. 13B is a picture taken with all pixels turnedon after storage at 85° C. for 1024 hours. It is found that the numberand the area of regions where pixel emission intensity is decreased aredrastically increased in the light emitting device, Comparative example6, having the above described structure, after 1024 hours at 85° C.,compared with the state before performing the storage test. Inparticular, the area of the region where emission intensity isdecreased, which existed across a plurality of pixels, is significantlyenlarged compared with the state before performing the storage test. Inthe structure of FIG. 13A or 13B, the etching stopper film is entirelyformed over the insulating layer. Therefore, water in the insulatinglayer cannot be sufficiently removed during the step of forming thelight emitting device. Further, the water remaining in the insulatinglayer seeps, in a concentrated manner, from a tiny pinhole in theetching stopper film or at the boundary with an electrode or the like.Therefore, the pixel located in the periphery of the portion where waterseeps is deteriorated. This is a conceivable reason for generating theregion where emission intensity is decreased and enlarging the region.

It is found that the deterioration of a light emitting element can beeffectively suppressed by using the structure of the invention asdescribed above.

This application is based on Japanese Patent Application serial No.2004-091710 filed in Japan Patent Office on Mar. 26, 2004, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light emitting device comprising: a pixelportion comprising a light emitting element and a first transistor; afirst wiring electrically connected to one of a source electrode and adrain electrode of the first transistor; a first protective circuitcomprising a first diode and a second diode wherein the first diode andthe second diode are electrically connected to the first wiring; asecond wiring electrically connected to a gate electrode of the firsttransistor; a second protective circuit comprising a third diode and afourth diode wherein the third diode and the fourth diode areelectrically connected to the second wiring.
 2. The light emittingdevice according to claim 1, wherein a gate electrode of the first diodeis electrically connected to a drain electrode or a source electrode ofthe first diode, and wherein a gate electrode of the second diode iselectrically connected to a drain electrode or a source electrode of thesecond diode.
 3. The light emitting device according to claim 1, whereina gate electrode of the third diode is electrically connected to a drainelectrode or a source electrode of the third diode, and wherein a gateelectrode of the fourth diode is electrically connected to a drainelectrode or a source electrode of the fourth diode.
 4. The lightemitting device according to claim 1, further comprising a secondtransistor electrically connected to the first transistor and the lightemitting element in the pixel portion.
 5. The light emitting deviceaccording to claim 1, further comprising a capacitor element in thepixel portion.
 6. The light emitting device according to claim 1,wherein the first wiring is a signal line.
 7. The light emitting deviceaccording to claim 1, wherein the second wiring is a scanning line. 8.The light emitting device according to claim 1, wherein the firsttransistor comprises a semiconductor film comprising silicon.
 9. Thelight emitting device according to claim 1, further comprising aninsulating layer between the first transistor and the light emittingelement, and a film selectively formed between the insulating layer andan electrode of the light emitting element, wherein the film is formedof a different material from the insulating layer.
 10. The lightemitting device according to claim 1, wherein the first wiring isdirectly connected to the one of the source electrode and the drainelectrode of the first transistor, and wherein the first diode and thesecond diode are directly connected to the first wiring.
 11. A lightemitting device comprising: a pixel portion comprising a light emittingelement and a first transistor wherein the light em itting elementcomprises an electroluminescent layer; a first wiring electricallyconnected to one of a source electrode and a drain electrode of thefirst transistor; a first protective circuit comprising a first diodeand a second diode wherein the first diode and the second diode areelectrically connected to the first wiring; a second wiring electricallyconnected to a gate electrode of the first transistor; a secondprotective circuit comprising a third diode and a fourth diode whereinthe third diode and the fourth diode are electrically connected to thesecond wiring, wherein the light emitting device is configured to emitlight to both sides of the electroluminescent layer.
 12. The lightemitting device according to claim 11, wherein a gate electrode of thefirst diode is electrically connected to a drain electrode or a sourceelectrode of the first diode, and wherein a gate electrode of the seconddiode is electrically connected to a drain electrode or a sourceelectrode of the second diode.
 13. The light emitting device accordingto claim 11, wherein a gate electrode of the third diode is electricallyconnected to a drain electrode or a source electrode of the third diode,and wherein a gate electrode of the fourth diode is electricallyconnected to a drain electrode or a source electrode of the fourthdiode.
 14. The light emitting device according to claim 11, furthercomprising a second transistor electrically connected to the firsttransistor and the light emitting element in the pixel portion.
 15. Thelight emitting device according to claim 11, further comprising acapacitor element in the pixel portion.
 16. The light emitting deviceaccording to claim 11, wherein the first wiring is a signal line. 17.The light emitting device according to claim 11, wherein the secondwiring is a scanning line.
 18. The light emitting device according toclaim 11, wherein the first transistor comprises a semiconductor filmcomprising silicon.
 19. The light emitting device according to claim 11,further comprising an insulating layer between the first transistor andthe light emitting element, and a film selectively formed between theinsulating layer and an electrode of the light emitting element, whereinthe film is formed of a different material from the insulating layer.20. The light emitting device according to claim 11, wherein the firstwiring is directly connected to the one of the source electrode and thedrain electrode of the first transistor, and wherein the first diode andthe second diode are directly connected to the first wiring.
 21. A lightemitting device comprising: a pixel portion comprising a light emittingelement and a first transistor wherein the light emitting elementcomprises an electroluminescent layer; a first wiring electricallyconnected to one of a source electrode and a drain electrode of thefirst transistor; a first protective circuit comprising a first diodeand a second diode wherein the first diode and the second diode areelectrically connected to the first wiring; a second wiring electricallyconnected to a gate electrode of the first transistor; a secondprotective circuit comprising a third diode and a fourth diode whereinthe third diode and the fourth diode are electrically connected to thesecond wiring, wherein the light emitting device is a top emission type.22. The light emitting device according to claim 21, wherein a gateelectrode of the first diode is electrically connected to a drainelectrode or a source electrode of the first diode, and wherein a gateelectrode of the second diode is electrically connected to a drainelectrode or a source electrode of the second diode.
 23. The lightemitting device according to claim 21, wherein a gate electrode of thethird diode is electrically connected to a drain electrode or a sourceelectrode of the third diode, and wherein a gate electrode of the fourthdiode is electrically connected to a drain electrode or a sourceelectrode of the fourth diode.
 24. The light emitting device accordingto claim 21, further comprising a second transistor electricallyconnected to the first transistor and the light emitting element in thepixel portion.
 25. The light emitting device according to claim 21,further comprising a capacitor element in the pixel portion.
 26. Thelight emitting device according to claim 21, wherein the first wiring isa signal line.
 27. The light emitting device according to claim 21,wherein the second wiring is a scanning line.
 28. The light emittingdevice according to claim 21, wherein the first transistor comprises asemiconductor film comprising silicon.
 29. The light emitting deviceaccording to claim 21, further comprising an insulating layer betweenthe first transistor and the light emitting element, and a filmselectively formed between the insulating layer and an electrode of thelight emitting element, wherein the film is formed of a differentmaterial from the insulating layer.
 30. The light emitting deviceaccording to claim 21, wherein the first wiring is directly connected tothe one of the source electrode and the drain electrode of the firsttransistor, and wherein the first diode and the second diode aredirectly connected to the first wiring.